1. Field of the invention
The invention relates to a method for determining the state of strength of winding overhangs of electric machines. The invention also relates to an arrangement for carrying out this method.
2. Discussion of Background
In electric machines, in particular generators having an output of more than 200 MVA, the winding overhang, that is to say the part of the winding which projects on both sides beyond the laminated core, must be configured such that it withstands the magnetic forces occurring during operation. These forces can assume a substantial magnitude in the case of a short circuit, and thus constitute a remarkable potential risk for the entire machine.
It is always possible in principle that the required mechanical pretensioning of the winding overhang assembly will decrease after a few years of operation. This is all the more probable the more short circuits a generator has undergone, for example. However, changes in the mechanical properties of the winding overhang assembly can also occur in the case of generators which are exposed to frequent load changes and the temperature fluctuations associated therewith.
A measured variable which is also influenced during operation by, inter alia, the mechanical properties of the winding overhang assembly is the winding overhang vibration, which can be measured locally in radial, tangential and axial directions using acceleration pickups. It is known to use the vibration accelerations measured at different points on the winding overhang circumference to determine the local vibration deflections or vibration amplitudes by twofold integration, and to monitor these parameters for limiting values. In this case, it is normally only the double line-frequency components of the winding overhang vibration which are considered. In normal operation, said winding overhang vibration is essentially excited by the symmetrical current system in the stator winding and, depending on the type of design of the winding overhang brackets, more or less also by the vibration of the stator laminated body.
Vibration monitoring proceeding according to this principle is known, for example, from "Modern Monitoring used in Life Management of Generator", Conference Proceedings September 1993, "Elektron" SAIEE-Journal. It is proposed herein to refer the vibration amplitude and its phase to a wave reference signal for each measuring optical sensor. A problem with this type of monitoring is assigning an increased deflection of the winding overhang vibration to the concrete cause, that is to say it is not possible to establish whether the excitation has increased or decreased, or whether the winding overhang assembly has become softer or harder, in terms of vibration.